Semiconductor device, manufacturing method thereof and imaging apparatus

ABSTRACT

A semiconductor device for converting incident light into an electric current includes a semiconductor substrate; an electrode embedded in the semiconductor substrate; an insulation film contacting the electrode in the semiconductor substrate; a first semiconductor region of a first conductivity type, a second semiconductor region of a second conductivity type and a third semiconductor region of the first conductivity type, formed sequentially in a depth direction from a side of a front face of the semiconductor substrate; and a fourth semiconductor region of the second conductivity type contacting the insulation film and the second semiconductor region. An impurity concentration of the fourth semiconductor region is greater than an impurity concentration of the second semiconductor region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The disclosures herein generally relate to a semiconductor device, amanufacturing method thereof and an imaging apparatus.

2. Description of the Related Art

A phototransistor having a bipolar structure has a feature of amplifyingan electric current according to a physical property, which the bipolarstructure has, on outputting a photo-electric current obtained at aphoto diode located between a collector and a base from an emitter.Therefore, sensitivity is enhanced with a smaller light-receiving areain a case where light intensity is low.

However, although the sensitivity in the case where light intensity islow is enhanced, the photo transistor has a difficult aspect in atreatment, such that careful handling is required for saturation of anoutput signal, since the photo-electric current increases, as the lightintensity becomes greater. Then, for example, Japanese Published PatentApplication No. 2013-187527 discloses a technique of varying sensitivityfor light intensity by varying a current amplification factor of a phototransistor, and thereby obtaining a proper sensitivity for each of thelight intensities.

SUMMARY OF THE INVENTION

It is a general object of at least one embodiment of the presentinvention to provide a semiconductor device, a manufacturing methodthereof and an imaging apparatus that substantially obviate one or moreproblems caused by the limitations and disadvantages of the related art.

In one embodiment, a semiconductor device for converting incident lightinto an electric current, includes a semiconductor substrate; anelectrode embedded in the semiconductor substrate; an insulation filmcontacting the electrode in the semiconductor substrate; a firstsemiconductor region of a first conductivity type, a secondsemiconductor region of a second conductivity type and a thirdsemiconductor region of the first conductivity type, formed sequentiallyin a depth direction from a side of a front face of the semiconductorsubstrate; and a fourth semiconductor region of the second conductivitytype contacting the insulation film and the second semiconductor region.An impurity concentration of the fourth semiconductor region is greaterthan an impurity concentration of the second semiconductor region.

In another embodiment, an imaging apparatus includes semiconductordevice for converting incident light into an electric current arrangedtwo-dimensionally. The semiconductor device includes a semiconductorsubstrate; an electrode embedded in the semiconductor substrate; aninsulation film contacting the electrode in the semiconductor substrate;a first semiconductor region of a first conductivity type, a secondsemiconductor region of a second conductivity type and a thirdsemiconductor region of the first conductivity type, formed sequentiallyin a depth direction from a side of a front face of the semiconductorsubstrate; and a fourth semiconductor region of the second conductivitytype contacting the insulation film and the second semiconductor region.An impurity concentration of the fourth semiconductor region is greaterthan an impurity concentration of the second semiconductor region.

In yet another embodiment, a manufacturing method of a semiconductordevice for converting incident light into an electric current includesforming an insulation film in a semiconductor substrate; embedding anelectrode so as to contact the insulation film in the semiconductorsubstrate; forming sequentially a first semiconductor region of a firstconductivity type, a second semiconductor region of a secondconductivity type and a third semiconductor region of the firstconductivity type in a depth direction from a side of a front face ofthe semiconductor substrate; and forming a fourth semiconductor regionof the second conductivity type so as to contact the insulation film andthe second semiconductor region. An impurity concentration of the fourthsemiconductor region is greater than an impurity concentration of thesecond semiconductor region.

According to the embodiment of the present invention, a semiconductordevice in which sensitivity for light intensity can be enhanced whilesuppressing an increase of dark current is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and further features of embodiments will become apparentfrom the following detailed description when read in conjunction withthe accompanying drawings, in which:

FIGS. 1A and 1B are diagrams illustrating an example of a main part of asemiconductor device according to a first embodiment;

FIG. 2 is a diagram illustrating an example of a diffusion profile ofthe semiconductor device according to the first embodiment;

FIGS. 3A and 3B are diagrams illustrating an example of a semiconductordevice according to a comparative example;

FIG. 4 is a diagram illustrating a threshold value shifting by providinga high impurity concentration region;

FIG. 5 is a diagram illustrating a current amplification factor changingaccording to a voltage applied to an electrode;

FIGS. 6A to 6C are diagrams illustrating an example of a manufacturingprocess of the semiconductor device according to the first embodiment;

FIGS. 7A to 7C are diagrams illustrating another example of themanufacturing process of the semiconductor device according to the firstembodiment;

FIGS. 8A and 8B are diagrams illustrating yet another example of themanufacturing process of the semiconductor device according to the firstembodiment;

FIGS. 9A to 9C are diagrams illustrating an example of a main part of asemiconductor device according to a first variation of the firstembodiment;

FIGS. 10A and 10B are diagrams illustrating an example of a main part ofa semiconductor device according to a second variation of the firstembodiment;

FIGS. 11A and 11B are diagrams illustrating an example of a main part ofa semiconductor device according to a third variation of the firstembodiment;

FIG. 12 is a diagram illustrating an example of a circuit configurationof a single imaging cell according to a second embodiment;

FIGS. 13A and 13B are diagrams illustrating an example of an imagingapparatus in which the single imaging cells are arrangedtwo-dimensionally according to the second embodiment;

FIGS. 14A and 14B are diagrams illustrating another example of theimaging apparatus in which the single imaging cells are arrangedtwo-dimensionally according to the second embodiment; and

FIG. 15 is a functional block diagram illustrating an example of theimaging apparatus according to the second embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the present invention will be describedwith reference to the accompanying drawings. In the respective drawings,the same reference numeral is assigned to the same component andduplicate explanation may be omitted.

First Embodiment Structure of Semiconductor Device

FIGS. 1A and 1B are diagrams illustrating an example of a main part of asemiconductor device according to a first embodiment. FIG. 1B is a planview, and FIG. 1A is a cross-sectional diagram cut along a line “A-A” inFIG. 1B. Meanwhile, in FIG. 1B, only a base region 12, a high impurityconcentration region 15, an insulation film 20 and an electrode 30 areillustrated, and a satin pattern is appropriately used as a matter ofconvenience.

A semiconductor device 1, as shown in FIGS. 1A and 1B, is, for example,a photo transistor including plural light receiving cells which performphoto-electric conversion for incident light. The semiconductor device 1includes a semiconductor substrate 10, the insulation film 20 and theelectrode 30. On a front face 10 a of the semiconductor substrate 10, aninterlayer insulation film 40 is formed. On the interlayer insulationfilm 40, a metal electrode 60 is formed. Since in the semiconductordevice 1 the respective light receiving cells function as phototransistors, the semiconductor device 1 may be referred to as a phototransistor array. However, although the present embodiment exemplifiesthe semiconductor device 1 including plural light receiving cells, thesemiconductor device 1 may be provided with a single light receivingcell.

Meanwhile, in the present embodiment, as a matter of convenience, a sideof the metal electrode 60 will be referred to as a front side or anupper side, and a side of a low resistance region 14, which will bedescribed later, will be referred to as a back side or a lower side.Moreover, a surface on a side of the metal electrode 60 of each partwill be referred to as a front face or an upper face, and a surface on aside of the low resistance region 14 will be referred to as a back faceor a lower face. However, the semiconductor device 1 can be used in astate of upside down, or may be arranged with an arbitrary angle.Moreover, a plan view indicates viewing an object in a normal directionto the front face 10 a of the semiconductor substrate 10, and a planarshape indicates a shape of an object viewed in the normal direction tothe front face 10 a of the semiconductor substrate 10.

The semiconductor substrate 10 is, for example, a silicone substrate. Onthe semiconductor substrate 10, an emitter region 11, a base region 12,and a collector region 13 are sequentially formed in a depth directionfrom a side of the front face 10 a of the semiconductor substrate 10. Alower side of the collector region 13 is, for example, referred to as anN⁺-type low resistance region 14. The emitter region 11 is electricallyconnected to the metal electrode 60 (emitter electrode) via a contact50. Meanwhile, a collector electrode may be provided on a back face ofthe low resistance region 14,

The emitter region 11 is, for example, N⁺-type. A thickness of theemitter region 11 may be set to about 0.2 to 0.4 μm, for example. Thebase region 12 is, for example, P-type. A thickness of the base region12 may be set to about 0.5 to 1.4 μm, for example. The collector region13 is, for example, N-type. A thickness of the collector region 13 maybe set to about 5 to 30 μm, for example.

Meanwhile, the emitter region 11 is a representative example of a firstsemiconductor region of a first conductivity type according to thepresent invention. Moreover, the base region 12 is a representativeexample of a second semiconductor region of a second conductivity typeaccording to the present invention. Moreover, the collector region 13 isa representative example of a third semiconductor region of the firstconductivity type according to the present invention. Here, the firstconductivity type means any one of the P-type or the N-type. The secondconductivity type means the P-type or the N-type, which is an oppositeconductivity type to the first conductivity type.

As shown in FIG. 2, an impurity concentration of the emitter region 11can be set to, for example, about 1×10²⁰ cm⁻³. An impurity concentrationof the base region 12 is inclined, i.e. a high impurity concentration ona side of the emitter region 11 and a low impurity concentration on aside of the collector region 13. The impurity concentration of the baseregion 12 may be, for example, about 5×10¹⁷ cm⁻³ just below the emitterregion 11, and about 5×10¹⁵ cm⁻³ on the side of the collector region 13.

Returning to FIG. 1, the electrode 30 is embedded in the semiconductorsubstrate 10 from a side of the front face 10 a. Moreover, theinsulation film 20 insulating the electrode 30 from the semiconductorsubstrate 10 is provided in contact with the electrode 30 in thesemiconductor substrate 10. The insulation film 20 may include, forexample, a silicon dioxide film or a silicon nitride film. A thicknessof the insulation film 20 may be set to, for example, about 10 to 40 nm.The electrode 30 may be made of, for example, resistance-reduced N-typepolysilicon having an impurity concentration of greater than or equal to1×10²⁰ cm⁻³. A width of the electrode 30 may be set to, for example,about 0.3 to 0.8 μm. A depth of the electrode 30 may be set to, forexample, about 4 to 20 μm.

In the present embodiment, the electrode 30 passes through the emitterregion 11 and the base region 12, and an apical part of the electrode 30reaches the collector region 13. Each of the regions partitioned by theelectrode 30 functions as a light receiving cell. That is, thesemiconductor device 1 is a photo transistor having a common collectorpotential, in which plural light receiving cells each havingconfigurations of extracting photo-electric current from an emitter sideare arranged. A width of the light receiving cell (interval betweenadjacent electrodes 30) may be set to, for example, about 3 to 20 μm.

In this way, the semiconductor device 1 has a vertical bipolarstructure, in which the emitter region 11 contacting the electrode 30via the insulation film 20, the base region 12 and the collector region13 are sequentially formed in the depth direction from the side of thefront face 10 a of the semiconductor substrate 10. According to theabove-described configuration, by applying a voltage to the electrode30, a region around the electrode 30 is influenced by an electric field,and especially a width of the base region 12 which is a quasi-neutralregion varies, and as a result a current amplification factor of thesemiconductor device 1 can be changed.

Moreover, the impurity concentration of the base region 12 is inclined,with a high impurity concentration on the side of the emitter region 11and a low impurity concentration on the side of the collector region 13.According to the above-described configuration, upon applying anelectric voltage to the electrode 30, a depletion layer occurring in thebase region 12 on the side of the collector region 13 becomes easy tospread from around the electrode 30 into the inside. Then, compared withthe case where the impurity concentration in the base region 12 isuniform, the change in the current amplification factor can be madegreater.

In the semiconductor device 1, in a region which is deeper than theemitter region 11 and shallower than the collector region 13, a highimpurity concentration region 15, which contacts the insulation layer 20and the base region 12, is arranged. The high impurity concentrationregion 15 has the same conductivity type as the base region 12 (P⁺-typein the present embodiment), and has greater impurity concentration thanthe greatest impurity concentration in the base region 12. The highimpurity concentration region 15 is a representative example of a fourthsemiconductor region of the second conductivity type according to thepresent invention.

The high impurity concentration region 15 may be arranged anywhere aslong as it contacts the insulation film 20 and the base region 12. Inthe present embodiment, a high impurity concentration region 15 having aplanar shape of a picture frame shape is arranged just below the emitterregion 11. In a plan view, inside the high impurity concentration region15 (central part of the light receiving cell), the base region 12 isarranged. In other words, in a plan view, the high impurityconcentration region 15 lies adjacent to the base region 12.

As shown in FIG. 2, the impurity concentration of the high impurityconcentration region 15 is preferably greater than or equal to ten timesthe greatest impurity concentration in the base region 12. For example,in a case where the impurity concentration in the base region 12 isgreater than or equal to 5×10¹⁶ cm⁻³ but less than or equal to 5×10¹⁷cm⁻³, the impurity concentration of the high impurity concentrationregion 15 is preferably greater than or equal to 5×10¹⁸ cm⁻³. This isbecause it becomes possible to make a threshold value of a parasitic MOS(Metal Oxide Semiconductor) transistor 90, which will be describedlater, greater.

Meanwhile, the threshold value of the parasitic MOS transistor 90 isdetermined based on the film thickness of the insulation film 20 betweenthe electrode 30 and the base region 12 and a base diffusiveconcentration of the base region 12 adjacent to the insulation film 20.Therefore, by arranging the high impurity concentration region 15 tomake the concentration in the base region 12 greater, a desiredthreshold value can be obtained. Moreover, also by changing the filmthickness of the insulation film 20, the threshold value can be changed.

However, since the high impurity concentration region 15 lowers thecurrent amplification factor upon photo-electric conversion, a widththereof is preferably smaller. Therefore, it is preferable to set aborderline width that can change the threshold value of the parasiticMOS transistor 90, i.e. a borderline distance that the electric fieldreaches upon applying the electric voltage to the electrode 30.

To give a specific example, in a case where the electric voltage appliedto the electrode 30 is about 5 V, taking account of a width of thedepletion layer which increases according to the application of theelectric voltage, the width W of the high impurity concentration region15 is preferably about 0.2 to 1.0 μm. In this case, since if the widthof the light receiving cell is about 3 to 20 μm, the base region 12resides just below the emitter region 11 in the central part of thelight receiving cell, decreasing of the current amplification factor canbe prevented.

Here, technical meaning of providing the high impurity concentrationregion 15 in the semiconductor device 1 will be explained with referenceto a comparative example. FIGS. 3A and 3B are diagrams illustrating anexample of a semiconductor device according to the comparative example.The semiconductor device 1X according to the comparative example isdifferent from the semiconductor device 1 according to the presentinvention (see FIGS. 1A and 1B) in that the high impurity concentrationregion 15 is not provided.

In a case of treating the semiconductor device 1X shown in FIG. 3A as aphoto transistor, a presence of a parasitic MOS transistor 90 show inFIG. 3B causes a problem. The parasitic MOS transistor is formed in anemitter region 11 contacting an electrode 30 via an insulation film 20,a base region 12 and a collector region 13. The emitter region 11becomes a source of the parasitic MOS transistor 90, and the collectorregion 13 becomes a drain of the parasitic MOS transistor 90. Moreover,the base region 12 becomes a channel of the parasitic MOS transistor 90.

In a case of applying an electric voltage to the electrode 30 in thesemiconductor device 1X in order to increase a current amplificationfactor, the parasitic MOS transistor 90 turns ON and an electric currentirrespective of a photo-electric current is added. There is a problemthat due to the electric current added by the parasitic MOS transistor90 turning ON, a dark current increases, and sensitivity under a lowintensity of illumination is reduced.

Especially, in a case of reducing the impurity concentration of the baseregion 12 in order to increase the current amplification factor, athreshold value of the parasitic MOS transistor 90 turning ON decreasesand the dark current increases. Moreover, in a case of reducing the sizeof the light receiving cell, since a ratio of the parasitic MOStransistor 90 occupying in the light receiving cell increases, due to aninfluence of an electric field occurring by applying the electricvoltage to the electrode 30, the threshold value of the parasitic MOStransistor 90 turning ON decreases and the dark current increases.

In this way, in the conventional semiconductor device 1X, although it ispossible to vary the current amplification factor by applying anelectric voltage to the electrode 30, it is not possible to control thethreshold value of the parasitic MOS transistor 90, and an increase ofdark current cannot be suppressed.

Then, in the semiconductor device 1 according to the first embodiment,by providing the high impurity concentration region 15, a channelconcentration of the parasitic MOS transistor 90 is made greater, andthe threshold value of the parasitic MOS transistor 90 is shifted tohigher. In this way, by making the threshold value of the parasitic MOStransistor 90 greater than the conventional one, the increase of darkcurrent can be suppressed.

FIG. 4 is a diagram illustrating the threshold value shifting byproviding the high impurity concentration region. FIG. 4 shows arelation between the electric voltage (abscissa) to be applied to theelectrode 30 and the dark current (ordinate) in the semiconductordevices 1 and 1X. Meanwhile, the axis of ordinate is a logarithmic axis.

Specifically, data in FIG. 4 are obtained by measuring the emittercurrent (dark current) while sweeping the electric voltage applied tothe electrode 30 under a condition where an electric voltage Vce=5 V isapplied between the emitter region 11 and the collector region 13 via anelectrode or the like. In FIG. 4, “Id(1)” represents a characteristic ofthe semiconductor device 1, and “Id(1X)” represents a characteristic ofthe semiconductor device 1X.

Meanwhile, in the semiconductor device 1, the impurity concentration ofthe high impurity concentration region 15 is set to be greater than orequal to ten times the greatest impurity concentration in the baseregion 12. The width W of the high impurity concentration region 15 isset to be about 0.2 to 1.0 μm with respect to the light receiving cell(10×10 μm²).

As shown by “Id(1X)” in FIG. 4, in the semiconductor device 1X which isnot provided with a high impurity concentration region 15, when theelectric voltage applied to the electrode 30 exceeds about 0.2 V, thedark current starts increasing. The dark current starts increasing,since the parasitic MOS transistor 90 turns ON. That is, in thesemiconductor device 1X, a threshold value at which the parasitic MOStransistor 90 turns ON is about 0.2 V.

On the other hand, as shown by “Id(1)” in FIG. 4, in the semiconductordevice 1 which is provided with the high impurity concentration region15, when the electric voltage applied to the electrode 30 is from 0 toabout 3.7 V, the dark current is less than or equal to a measurementlimit (less than or equal to 1×10-11 A). Then, when the electric voltageapplied to the electrode 30 exceeds about 3.7 V, the dark current startsincreasing. That is, in the semiconductor device 1, the threshold valueat which the parasitic MOS transistor 90 turns ON is about 3.7 V, whichis shifted from the semiconductor device 1X to the higher side of thethreshold value by about 3.5 V.

In this way, it is found that in the semiconductor device 1, which isprovided with the high impurity concentration region 15, the thresholdvalue at which the parasitic MOS transistor 90 turns ON becomes greater(shifted to the higher side), compared with the semiconductor device 1X,which is not provided with the high impurity concentration region 15.

FIG. 5 is a diagram illustrating a current amplification factor changingaccording to a voltage applied to an electrode, and shows a relationbetween luminance (abscissa) and photo-electric current (ordinate) in acase of applying a predetermined electric voltage to the electrode 30 ofthe semiconductor device 1. Meanwhile, the axis of abscissa and the axisof ordinate are logarithmic axes.

Specifically, data in FIG. 5 are obtained by measuring the emittercurrent (photo-electric current) obtained by irradiating the lightreceiving cell (10×10 μm²) with halogen light while fixing the electricvoltage applied to the electrode 30 to a predetermined electric voltageunder a condition where an electric voltage Vce=5 V is applied betweenthe emitter region 11 and the collector region 13 via an electrode orthe like. Meanwhile, in FIG. 5, “O”, “□”, “Δ” and “X” representcharacteristics in a case where the predetermined electric voltageapplied to the electrode 30 is set to 4 V, 3.5 V, 3 V and 0 V,respectively.

Meanwhile, in the semiconductor device 1, the impurity concentration ofthe high impurity concentration region 15 is set to be greater than orequal to ten times the greatest impurity concentration in the baseregion 12. The width W of the high impurity concentration region 15 isset to be about 0.2 to 1.0 μm.

As shown in FIG. 5, in the semiconductor device 1, by changing theelectric voltage applied to the electrode 30, the photo-electric currentvaries with respect to the luminance. In this way, it is confirmed thatin the semiconductor device 1, even if the high impurity concentrationregion 15 is provided, the current amplification factor is changed bythe electric voltage applied to the electrode 30, and thereby thephoto-electric current can be changed.

[Manufacturing Method of Semiconductor Device]

FIGS. 6A to 8B are diagrams illustrating an example of a manufacturingprocess of the semiconductor device according to the first embodiment,and show a cross section corresponding to FIG. 1A.

First, as shown in FIG. 6A, for example, a silicone substrate providedwith an N-type epitaxial layer 190 with an electrical resistivity of 1Ωcm on an Ni-type low-resistivity region 14 with an electricalresistivity of 6 mΩcm is prepared. A thickness of the epitaxial layer190 may be set to an arbitrary value according to an absorption rate fora wavelength of a light source.

Next, as shown in FIG. 6B, in order to embed the electrode 30, a trench300 is formed in the epitaxial layer 190 penetrating through a regionwhich will be the emitter region 11 and the base region 12 into a regionwhich will be the collector region 13. The trench 300 may be formed by,for example, dry etching. A width of the trench 300 may be, for example,about 0.3 to 0.8 μm. A depth of the trench 300 may be, for example,about 4 to 20 μm.

Next, as shown in FIG. 6C, an insulation film 20 with a thickness ofabout 20 nm is formed on a bottom face and on an internal face of thetrench 300. Then, the electrode 30 is formed inside the trench 300 viathe insulation film 20. Specifically, at first, for example, aninsulation film 20 (silicon dioxide film) with a thickness of about 20nm, is formed by thermal oxidization method on the bottom face and onthe internal face of the trench 300 and on a front face of the epitaxiallayer 190. However, a silicon nitride film or the like may be used forthe insulation film 20.

Then, for example, polysilicon is deposited inside the trench 300 and onthe front face of the epitaxial layer 190, on which the insulation film20 is formed, by CVD method or sputtering method. Afterwards, byremoving the polysilicon formed on the front face using an etch back,and leaving only inside the trench 300, the electrode 30 formed ofpolysilicon is prepared. Meanwhile, in order to prepare the electrode 30with a small electric resistance, it is preferable to depositpolysilicon in which phosphorous impurity is saturated.

Next, as shown in FIG. 7A, in order to form a P-type base region 12,P-type impurity 120 is injected. Specifically, for example, boron isinjected as the P-type impurity 120, at an acceleration energy of 30 KeVand an injected dose of 3.2×10¹³ cm⁻².

Then, as shown in FIG. 7B, for example, by performing heat treatment atabout 1150° C. for about 50 minutes, the P-type impurity 120 isthermally diffused and activated, and thereby a base region 12 with adepth of about 1.5 mm is formed. Meanwhile, a layer below the baseregion 12 is the collector region 13. The depth and an impurityconcentration may be set to arbitrary values, with which the currentamplification factor is stable, according to an absorption rate for awavelength of the light source

Next, as shown in FIG. 7C, a P-type impurity 150 is injected in order toform a P⁺-type high impurity concentration region 15 contacting theinsulation film 20 and the base region 12 just below the region whichwill be the emitter region 11. Specifically, for example, boron isinjected as the P-type impurity 150, at 180 KeV and 1×10¹³ cm⁻².

Next, as shown in FIG. 8A, in order to form an N⁺-type emitter region11, N-type impurity 110 is injected. Specifically, for example,phosphorus is injected as the N-type impurity 110, at 50 KeV and 6×10¹³cm⁻².

Then, as shown in FIG. 8B, for example, by performing heat treatment atabout 920° C. for about 40 minutes, the N-type impurity 110 and theP-type impurity 150 are thermally diffused and activated, and therebythe emitter region 11 is formed on a side of a front face 10 a and thehigh impurity concentration region 15 is formed just below the emitterregion 11. Meanwhile, the diffusion is preferably shallow, since theemitter region 11 prevents light from being absorbed.

Afterwards, on the front face 10 a of the semiconductor substrate 10, aninterlayer insulation film 40 is formed by the CVD method or the like, acontact 50 connected to the emitter region 11 is formed in theinterlayer insulation film 40, and further a metallic electrode 60connected to the contact 50 is formed on the interlayer insulation film40. Moreover, the polysilicon forming the electrode 30 is extended froma periphery of a region where each of the light receiving cells isformed, and forms an extraction wiring of the electrode 30. Meanwhile,since the metallic electrode 60 shields light, the metallic electrode 60is preferably arranged on the electrode 30 to the extent possible.According to the respective above-described processes, the semiconductordevice 1 as shown in FIGS. 1A and 1B is completed.

In this way, in the semiconductor device 1 according to the firstembodiment, the electrode 30 is embedded in the semiconductor substrate10 via the insulation film 20, and the emitter region 11 contacting theelectrode 30, the base region 12 and the collector region 13 aresequentially formed in the depth direction from the side of the frontface 10 a of the semiconductor substrate 10. According to theabove-described configuration, by applying a voltage to the electrode30, a width of the base region 12 varies, and as a result a currentamplification factor of the semiconductor device 1 can be changed.(Sensitivity for light intensity can be enhanced.)

Moreover, in a region deeper than the emitter region 11 and shallowerthan the collector region 13, the high impurity concentration region 15,whose impurity concentration is greater than that of the base region 12and which has the same conductivity type as the base region 12 isarranged contacting the insulation film 20 and the base region 12.According to the above-described configuration, a channel concentrationof a parasitic MOS transistor 90 formed on the semiconductor substrate10 is made greater, and a threshold value of the parasitic MOStransistor 90 is shifted to higher. As a result, conventional one, theincrease of dark current can be suppressed in the semiconductor device1.

The technique disclosed in Japanese Published Patent Application No.2013-187527 has a problem that in a case of increasing a currentamplification factor and enhancing sensitivity for light intensity, adark current increases.

On the other hand, in the semiconductor device 1 according to thepresent embodiment, the sensitivity for light intensity can be enhancedwhile suppressing the increase of dark current.

<First Variation of First Embodiment>

In the first variation of the first embodiment, a high impurityconcentration region arranged at a position different from the firstembodiment will be illustrated. Meanwhile, in the first variation of thefirst embodiment, an explanation of the same component as the embodimentwhich has already been explained may be omitted.

FIGS. 9A to 9C are diagrams illustrating an example of a main part of asemiconductor device according to a first variation of the firstembodiment, and show cross sections corresponding to FIG. 1A.

In a semiconductor device 1A shown in FIG. 9A, a high impurityconcentration region 15A contacts only an insulation film 20 and a baseregion 12 without contacting an emitter region 11 and a collector region13.

In order to form the high impurity concentration region 15A, anacceleration voltage has only to be made greater in the process shown inFIG. 7C in the first embodiment so that an injection position of aP-type impurity 150 becomes deeper. For example, by injecting boron asthe P-type impurity (at 400 KeV and 1×10¹³ cm⁻²), the high impurityconcentration region 15A can be formed at the position shown in FIG. 9A.

Since also in the configuration shown in FIG. 9A, a channelconcentration of a parasitic MOS transistor 90 can be made greater as inthe configuration shown in FIGS. 1A and 1B, a threshold value of theparasitic MOS transistor 90 can be made greater and an increase of darkcurrent can be suppressed.

In a semiconductor device 1B shown in FIG. 9B, a high impurityconcentration region 15B is provided just above the collector region 13so as to contact the insulation film 20 and the base region 12.

In order to form the high impurity concentration region 15B, theacceleration voltage has only to be made greater than in the case ofFIG. 9A in the process shown in FIG. 7C in the first embodiment so thatthe injection position of the P-type impurity 150 becomes deeper. Forexample, by injecting boron as the P-type impurity (at 1000 KeV and1×10¹³ cm⁻²), the high impurity concentration region 15B can be formedat the position shown in FIG. 9B.

Since also in the configuration shown in FIG. 9B, the channelconcentration of the parasitic MOS transistor 90 can be made greater asin the configuration shown in FIGS. 1A and 1B, the threshold value ofthe parasitic MOS transistor 90 can be made greater and the increase ofdark current can be suppressed.

In the semiconductor devices 1, 1A and 1B, the high impurityconcentration regions 15, 15A and 15B are provided in parts which aredeeper than the emitter region 11 and shallower than the collectorregion 13 in the depth direction, respectively. On the other hand, inthe semiconductor device 13 shown in FIG. 9C, the high impurityconcentration region 15C is provided in an entirety which is deeper thanthe emitter region 11 and shallower than the collector region 13 in thedepth direction.

That is, in the semiconductor device 1C, the high impurity concentrationregion 15C is arranged from just below the emitter region 11 to justabove the collector region 13 contacting the insulation film 20 and thebase region 12. In other words, the high impurity concentration region15C having almost the same thickness as the base region 12 is providedbetween the insulation film 20 and the base region 12.

In order to form the high impurity concentration region 15C, theacceleration voltage has only to be changed to perform a multistageinjection of the P-type impurity 150 in the process shown in FIG. 7C inthe first embodiment so that the concentration becomes greater justbelow the emitter region, in a region further below it and just abovethe collector region 13. For example, by performing injection of boron(at 1000 KeV and 1×10¹³ cm⁻²), injection of boron (at 400 KeV and 1×10¹³cm⁻²) and injection of boron (at 180 KeV and 1×10¹³ cm⁻²) in series, thehigh impurity concentration region 15C can be formed at the positionshown in FIG. 9C.

Since also in the configuration shown in FIG. 9C, the channelconcentration of the parasitic MOS transistor 90 can be made greater asin the configuration shown in FIGS. 1A and 1B, the threshold value ofthe parasitic MOS transistor 90 can be made greater and the increase ofdark current can be suppressed.

In this way, as long as the high impurity concentration region contactsthe insulation film 20 and the base region 12, wherever the highimpurity concentration region is arranged, the threshold value of theparasitic MOS transistor 90 becomes greater and the increase of darkcurrent can be suppressed.

However, for increasing the threshold value of the parasitic MOStransistor 90, it is the most effective to arrange the high impurityconcentration region on a source side (a side of the emitter region 11)of the parasite MOS transistor 90. Moreover, the high impurityconcentration region is preferably arranged as close as possible to thefront face 10 a of the semiconductor substrate 10 in manufacturing thesemiconductor device 1 to the extent of requiring relatively lowacceleration voltage upon injecting impurity. From the above-describedstandpoint, the semiconductor device 1 in which the high impurityconcentration region 15 is arranged just below the emitter region 11(See FIGS. 1A and 1B) can be said to be the most preferableconfiguration.

<Second Variation of First Embodiment>

In the second variation of the first embodiment, an electrode 30penetrating the collector region 13 will be illustrated. Meanwhile, inthe second variation of the first embodiment, an explanation of the samecomponent as the embodiment which has already been explained may beomitted.

FIGS. 10A and 10B are diagrams illustrating an example of a main part ofa semiconductor device according to the second variation of the firstembodiment. FIG. 10B is a plan view and FIG. 10A is a cross-sectionalview cut along a line B-B in FIG. 10B. However, in FIG. 10B, only thebase region 12, the high impurity concentration region 15, theinsulation film 20 and the electrode 30 are shown, and a satin patternis appropriately used as a matter of convenience. Meanwhile, anarrangement of the emitter region 11, the base region 12, the collectorregion 13, an ohmic region 16 and a high concentration base region 17 isthe same as FIG. 11B, which will be described later.

In a semiconductor device 1D shown in FIGS. 10A and 10B, a P-typesubstrate is used for a semiconductor substrate 10D. In thesemiconductor device 1D, an electrode 30 passes through the emitterregion 11 (e.g. Ni-type), the base region 12 (e.g. P-type) and thecollector region 13 (e.g. N-type). An apical part of the electrode 30reaches a P-type region 14D.

According to the above-described configuration, in each of the lightreceiving cells, the emitter regions 11 are mutually separated, the baseregions 12 are mutually separated, and the collector regions 13 aremutually separated (i.e. the collector regions 13 in the respectivelight receiving cells are electrically independent from each other).Meanwhile, in the same way as the semiconductor device 1 (see FIGS. 1Aand 1B), just below the emitter region 11, the high impurityconcentration region 15 is arranged contacting the insulation film 20and the base region 12.

Moreover, the emitter region 11, the base region 12 and the collectorregion 13 exist facing the front face 10 a of the semiconductorsubstrate 10D. Then, the emitter region 11 is electrically connectedwith a metallic electrode 60 (emitter electrode) via a contact 50.Moreover, on a front face of the collector region 13, an Ni-type ohmicregion 16 is arranged. The ohmic region 16 is connected to a metallicelectrode 70 (collector electrode) via the contact 50.

In other words, a horizontal bipolar structure exists on a side of thefront face 10 a of the semiconductor substrate 10D along with a verticalbipolar structure in the depth direction of the semiconductor substrate10D. In the horizontal bipolar structure, variation of a currentamplification factor for collector electric current is great. Then, inorder to suppress the variation of the current amplification factor forcollector electric current, it is preferable to arrange a P⁺-type highconcentration base region 17 is preferably arranged at a positionlocated separately from the emitter region 11 on the front face of thebase region 12. For example, the high concentration base region 17 witha concentration of greater than or equal to 1×10¹⁹ cm⁻³ can be arrangedat a position which is separated from the emitter region 11 by about 1μm on the front face of the base region 12.

According to the configuration shown in FIGS. 10A and 10B, in thesemiconductor device 1D, different electric voltages can be applied tothe metallic electrode 60 (emitter electrode) and the metallic electrode70 (collector electrode) in each of the light receiving cells.Therefore, since an emitter electric potential and a collector electricpotential can be set freely and it is possible to select the metallicelectrode 60 (emitter electrode) or the metallic electrode 70 (collectorelectrode) to acquire an output signal, a degree of freedom upon thecircuit operating can be enhanced. Meanwhile, an effect by forming thehigh impurity concentration region 15 is the same as the firstembodiment.

<Third Variation of First Embodiment>

In a third variation of the first embodiment, an SOI (Silicon OnInsulator) substrate used for the semiconductor substrate will beillustrated. Meanwhile, in the third variation of the first embodiment,an explanation of the same component as the embodiment which has alreadybeen explained may be omitted.

FIGS. 11A and 11B are diagrams illustrating an example of a main part ofa semiconductor device according to the third variation of the firstembodiment. FIG. 11B is a plan view and FIG. 11A is a cross-sectionaldiagram cut along a line C-C in FIG. 11B. However, in FIG. 11B, only anemitter region 11, a base region 12, a collector region 13, an ohmicregion 16, a high concentration base region 17, an insulation film 20and an electrode 30 are shown, and a satin pattern is appropriately usedas a matter of convenience. Meanwhile, an arrangement of a high impurityconcentration region 15 is the same as above-described FIG. 10B.

In a semiconductor device 1E shown in FIGS. 11A and 11B, for asemiconductor substrate 10E, an SOI substrate in which a BOX (BuriedOxide) oxide film 18 with a thickness of about 1 mm and a silicon activelayer are arranged in series on a P-type silicon substrate 14E is used.Then, in the silicon active layer arranged on the BOX oxide film 18, theemitter region 11, the base region 12, the collector region 13, the highimpurity concentration region 15, the ohmic region 16 and the highconcentration base region 17 of the same structure as the semiconductordevice 1D (see FIGS. 10A and 10B) are arranged. Below the collectorregion 13, the BOX oxide film 18 contacting the insulation film 20 isarranged.

In the semiconductor device 1E, the electrode 30 passes through theemitter region 11, the base region 12 and the collector region 13, andthe insulation film 20 coating an apical part of the electrode 30reaches the BOX oxide film 18. According to the above-describedconfiguration, in each of the light receiving cells, the emitter regions11 are mutually separated, the base regions 12 are mutually separatedand the collector regions 13 are mutually separated (i.e. the collectorregions 13 in the respective light receiving cells are electricallyindependent from each other).

The semiconductor device 1E has a structure in which adjacent lightreceiving cells are mutually insulated and separated completely,different from the structure of the semiconductor devices 1 to 1C, inwhich the collector region 13 is common, or from the structure of thesemiconductor device 1D, in which the P-type region 14D and thecollector region 13 form a PN junction. As a result, since diffusion ofelectric charges occurring due to injection of light is restricted onlyinside each of the light receiving cells and does not move to anadjacent light receiving cell, color mixture can be suppressed in a casewhere the semiconductor device 1E is used for an imaging apparatus.Meanwhile, an effect by forming the high impurity concentration region15 is the same as the first embodiment.

Second Embodiment

In a second embodiment, an imaging apparatus in which the semiconductordevice 1 according to the first embodiment is used for a phototransistor will be illustrated.

Meanwhile, in the second embodiment, an explanation of the samecomponent as the embodiment which has already been explained may beomitted.

FIG. 12 is a diagram illustrating an example of a circuit configurationof a single imaging cell. As shown in FIG. 12, the single imaging cell 2includes a light receiving cell 10 ce(photo transistor) of thesemiconductor device 1 and a MOS switch for reading 200 which is turnedON/OFF according to an electric voltage applied to an IN₂ terminal.While the imaging cell 2 is irradiated with light, in a case of turningoff the MOS switch 200, a base region 12 of the light receiving cell 10ce accumulate electric charges. By turning on the MOS switch 200, anoutput electric current (photo-electric current) amplified with acurrent amplification factor which the light receiving cell 10 cepossesses can be extracted from an OUT terminal of the MOS switch 200.

In a case where light intensity for irradiation is low and the outputelectric current to be extracted is small, sensitivity for the lightintensity can be enhanced by applying an electric voltage to the IN₁terminal (an electrode 30 adjacent to the light receiving cell 10 ce) soas to increase the current amplification factor and thereby increasingthe output electric current. Conversely, in a case where the lightintensity becomes great and the output electric current is saturated, byapplying an electric voltage to the IN₁ terminal so as to decrease thecurrent amplification factor and lowering the sensitivity for the lightintensity, an accurate output electric current corresponding to thelight intensity can be obtained.

Meanwhile, the MOS switch 200 can be formed on the semiconductorsubstrate 10 included in the semiconductor device 1 and adjacent to thelight receiving cell 10 ce, for example.

FIGS. 13A and 13B are diagrams illustrating an example of an imagingapparatus in which the single imaging cells are arrangedtwo-dimensionally. FIG. 13A is a simplified block diagram illustratingan example of a circuit configuration of an imaging apparatus 3. FIG.13B is a plan view illustrating an example of an arrangement of thelight receiving cells 10 ce and the MOS switches 200 in the imagingapparatus 3.

In the imaging apparatus 3, for example, the imaging cells are arrangedin 3 columns by 3 rows. In the imaging apparatus 3, the MOS switches 200included in the imaging cell 2 are provided with a common terminal foreach column (IN₂₋₁, IN₂₋₂ or IN₂₋₃). Moreover, the imaging apparatus 3includes a MOS switch, which selects a row based on electric voltagesapplied to the IN₃₋₁ to IN₃₋₃ terminals, and sense amplifiers foramplifying outputs of the MOS switches 210.

In the imaging apparatus 3, an address of the light receiving cell 10 ce(which column and which row) is selected at a constant frequency byusing the MOS switches 200 and 210. Then, an output electric currentfrom the selected light receiving cell 10 ce is amplified by the senseamplifier 220 and outputted from an OUT₁ terminal to an OUT₃ terminal,is subjected to data processing, and thereby a two-dimensional image canbe obtained.

In the imaging apparatus 3, since the electrode 30 is made common, andcurrent amplification factors of the respective imaging cells 2 arecontrolled by a signal inputted to the IN₁ terminal (an electrode 30common to all the light receiving cells 10 ce), the currentamplification factors of all the light receiving cells 10 ce of thesemiconductor device 1 can be changed collectively.

According to the above-described configuration, in a case where lightintensity of the whole screen is low and the maximum signal level ofoutput electric current is low, entire output signal can be enlarged byincreasing a current amplification factor. Moreover, in a case where anaccurate output signal cannot be obtained from the sense amplifier 220or the output signal is saturated due to an excessively great lightintensity, the output signal can be reduced and changed to an accurateoutput signal by decreasing the current amplification factor.

FIGS. 14A and 14B are diagrams illustrating another example of theimaging apparatus in which the single imaging cells are arrangedtwo-dimensionally. FIG. 14A is a simplified block diagram illustratingan example of a circuit configuration of an imaging apparatus 4. FIG.14B is a plan view illustrating an example of an arrangement of thelight receiving cells 10 ce and the MOS switches 200 in the imagingapparatus 4. In the imaging apparatus 4 shown in FIGS. 14A and 14B, anindependent electrode 30 (IN₁₋₁₁ or the like) is provided in everysingle imaging cell 2.

According to the above-described configuration, the currentamplification factor of each of the imaging cells 2 can be setindependently by applying a predetermined electric voltage to an INterminal (an electrode 30 specific to each of the light receiving cells10 ce) corresponding to the respective imaging cell 2. Therefore, bydetecting an excessively bright part or an excessively dark part in theimage and feeding it back, an output signal is corrected and smoothed,and thereby image quality can be enhanced. Moreover, as in a specialphotographing, a contrast of an image of a part which is desired to behighlighted can be changed.

FIG. 15 is a functional block diagram illustrating an example of theimaging apparatus. In the example of FIG. 15, an electric voltagegenerator 230 for generating an electric voltage for which a currentamplification factor is set to be quintupled or an electric voltagegenerator 240 for generating an electric voltage for which the currentamplification factor is set to be half is properly selected based on anoutput from a comparator circuit 250.

The comparator circuit 250 is a circuit which monitors a maximum valueof an output signal outputted from lots of light receiving cells 10 ce(photo transistors) via the sense amplifier 220, and controls anelectric voltage applied to the electrode 30 based on a result of themonitor. Meanwhile, the output signal outputted via the sense amplifier220 is a photo-electric current of an electric voltage obtained byconverting the photo-electric current. In the comparator circuit 250, alevel “E” which is one-tenth of a predetermined photo-electric current“D” and a level which reaches the predetermined photo-electric current“D” are set as threshold values in advance.

For example, it is assumed that in an initial state the currentamplification factor of the light receiving cell 10 ce is set to betwice. In this case, in a case where the output signal from the senseamplifier 220 is lower than the level “E”, the electric voltagegenerator 230 is selected according to an output from the comparatorcircuit 250. Then, based on an electric voltage generated by theelectric voltage generator 230, the current amplification factor of thelight receiving cell 10 ce becomes quintupled.

Moreover, in a case where the output signal from the sense amplifier 220is greater than the level “D”, the electric voltage generator 240 isselected according to the output from the comparator circuit 250. Then,based on the electric voltage generated by the electric voltagegenerator 240, the current amplification factor of the light receivingcell 10 ce becomes a half. Moreover, in a case where the output signalfrom the sense amplifier 220 is greater than or equal to the level “E”but less than or equal to the level “D”, neither the electric voltagegenerator 230 nor the electric voltage generator 240 is selected, andthe current amplification factor becomes twice.

According to the above-described configuration, even in a case ofhandling light intensities which are significantly different from eachother, it is possible to put output signals from the sense amplifier 220into an approximately fixed range. Meanwhile, in an example of FIG. 15,two kinds of threshold values for the comparator circuit 250 andelectric voltages to be set by the electric voltage generator (electricvoltage generators 230 and 240) are assumed. However, by segmentalizingthe threshold values and electric voltages to be set into three or morekinds, further detailed correction becomes possible.

Meanwhile, since the semiconductor device 1 according to the firstembodiment is used in the imaging apparatuses 3 and 4 according to thesecond embodiment, an increase of dark current can be suppressed.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

For example, in the first embodiment, the emitter regions 11 of therespective light receiving cells partitioned by the electrode 30 may beelectrically connected and be made common, and thereby the semiconductordevice 1 may be made to be a large-area single photo transistor. Also inthis case, the same effect as the first embodiment is provided.

Moreover, in the second or third variation of the first embodiment, inthe same way as in the first variation of the first embodiment, a highimpurity concentration region may be arranged at a position other thanthe position just below the emitter region 11.

Moreover, in the second embodiment, any one of the semiconductor devices1A to 1E according to the first to third variations of the firstembodiment may be used instead of the semiconductor device 1.

Moreover, the conductivity types of the emitter region 11, the baseregion 12, the collector region 13, the high impurity concentrationregion 15 and the like may be opposite to those illustrated in therespective embodiments.

The present application is based on and claims the benefit of priorityof Japanese Priority Application No. 2014-228766 filed on Nov. 11, 2014,the entire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device for converting incidentlight into an electric current, comprising: a semiconductor substrate;an electrode embedded in the semiconductor substrate; an insulation filmcontacting the electrode in the semiconductor substrate; a firstsemiconductor region of a first conductivity type, a secondsemiconductor region of a second conductivity type and a thirdsemiconductor region of the first conductivity type, formed sequentiallyin a depth direction from a side of a front face of the semiconductorsubstrate; and a fourth semiconductor region of the second conductivitytype contacting the insulation film and the second semiconductor region,an impurity concentration of the fourth semiconductor region beinggreater than an impurity concentration of the second semiconductorregion.
 2. The semiconductor device as claimed in claim 1, wherein theimpurity concentration of the fourth semiconductor region is greaterthan or equal to ten times the greatest impurity concentration of thesecond semiconductor region.
 3. The semiconductor device as claimed inclaim 1, wherein the fourth semiconductor region is arranged between thefirst semiconductor region and the third semiconductor region so as tobe separated from at least one of the first semiconductor region and thethird semiconductor region.
 4. The semiconductor device as claimed inclaim 3, wherein the fourth semiconductor region is arranged so as tocontact the first semiconductor region.
 5. The semiconductor device asclaimed in claim 1, wherein the fourth semiconductor region is arrangedso as to contact the first semiconductor region and the thirdsemiconductor region.
 6. The semiconductor device as claimed in claim 1,wherein the electrode passes through the first semiconductor region, thesecond semiconductor region and the third semiconductor region.
 7. Thesemiconductor device as claimed in claim 1, further comprising a BOX(Buried Oxide) film arranged below the third semiconductor region so asto contact the insulation film, wherein the semiconductor substrate isan SOI (Silicon On Insulator) substrate.
 8. An imaging apparatuscomprising the semiconductor device as claimed in claim 1 arrangedtwo-dimensionally.
 9. The imaging apparatus as claimed in claim 8,further comprising a circuit configured to monitor an electric currentoutputted from the semiconductor device or an electric voltage obtainedby converting the electric current, and to control an electric voltageto be applied to the electrode based on the monitored electric currentor the monitored electric voltage.
 10. A manufacturing method of asemiconductor device for converting incident light into an electriccurrent, the method comprising: forming an insulation film in asemiconductor substrate; embedding an electrode so as to contact theinsulation film in the semiconductor substrate; forming sequentially afirst semiconductor region of a first conductivity type, a secondsemiconductor region of a second conductivity type and a thirdsemiconductor region of the first conductivity type in a depth directionfrom a side of a front face of the semiconductor substrate; and forminga fourth semiconductor region of the second conductivity type so as tocontact the insulation film and the second semiconductor region, animpurity concentration of the fourth semiconductor region being greaterthan an impurity concentration of the second semiconductor region. 11.The manufacturing method of the semiconductor device as claimed in claim10, wherein the impurity concentration of the fourth semiconductorregion is made greater than or equal to ten times the greatest impurityconcentration of the second semiconductor region.
 12. The manufacturingmethod of the semiconductor device as claimed in claim 10, wherein thefourth semiconductor region is formed between the first semiconductorregion and the third semiconductor region so as to be separated from atleast one of the first semiconductor region and the third semiconductorregion.
 13. The manufacturing method of the semiconductor device asclaimed in claim 12, wherein the fourth semiconductor region is arrangedso as to contact the first semiconductor region.
 14. The manufacturingmethod of the semiconductor device as claimed in claim 10, wherein thefourth semiconductor region is arranged so as to contact the firstsemiconductor region and the third semiconductor region.
 15. Themanufacturing method of the semiconductor device as claimed in claim 10,wherein the electrode passes through the first semiconductor region, thesecond semiconductor region and the third semiconductor region.
 16. Themanufacturing method of the semiconductor device as claimed in claim 10,further comprising forming a BOX film below the third semiconductorregion so as to contact the insulation film, wherein the semiconductorsubstrate is an SOI substrate.